The present invention relates to a process for treating a surface of a copper foil for a printed circuit board, particularly to a process for treating the surface of the copper foil wherein a zinc layer and a copper layer covering the zinc layer are formed on the surface of the copper foil and if necessary, an additional zinc layer is formed and a chromate layer is formed on these surfaces, whereby the reduction of the bond strength between the copper foil and a substrate by application of heat, and also the reduction of the corrosion resistance to hydrochloric acid are prevented.
Generally when a printed circuit board is manufactured, there is used a copper-clad laminate in which a copper foil is bonded to a supporting substrate.
In the case of manufacturing this printed circuit board by the use of such a copper-clad laminate, the copper-clad laminate is subjected to punching first, and the insides of the holes are activated. Then, copper is uniformly deposited by means of nonelectrolytic plating and further, the copper is electroplated to augment its thickness. Then, after applying a photoresist to the surface of the copper foil, pattern etching is conducted to obtain the objective circuit board. During the activating treatment of the inside of holes and the pattern etching treatment in this process, a bonding boundary layer between the copper foil and the substrate is exposed to a solution containing hydrochloric acid, and after performing the pattern etching treatment, the copper foil is immersed several times in the various plating solutions and acid cleaning solutions as pretreatment in the case of conducting various plating steps.
Accordingly, in the case that the above-stated step is conducted, the bonding boundary layer between the copper foil and the substrate is sometimes corroded by acids to deteriorate the bond strength. Also, in the case of bonding the copper foil to the substrate, it is heated to bond them under a pressure. Therefore, the stability of bond strength between the copper foil and the substrate becomes important in the step for preparing the printed circuit board.
For this reason, to the surface by which copper foil is bonded to a substrate, surface treatment is hitherto performed by means of electrodeposition which is performed by the use of copper foil as a cathode. That is, in order to enhance the bond strength between the copper foil and the substrate, a granular copper layer has been electrically deposited on the surface of the copper foil by burned plating, to thereby form a roughened surface. To prevent falling off thereof, a smooth copper plating is applied, at a limiting current density or less and to said electroplating surface is applied coating with a metal such as zinc, zinc alloys and the like for the purpose of rust resistance and heat resistance.
However, the above-stated copper foil covered with zinc or zinc alloys has not been provided with a satisfactory resistance to hydrochloric acid for the printed circuit board. For example, when the section of the copper foil produced by punching or pattern etching of the copper-clad laminate is exposed to a solution containing hydrochloric acid in the step for producing the printed circuit board, corrosion due to acid is caused at the boundary between said zinc type corrosion resistant layer and the substrate, resulting in problems that bond strength becomes unstable. Also, there is a problem on safety because any practical method other than a cyanide bath does not exist for electroplating brass which is an alloy of copper and zinc.